The invention relates to a process for coating a substrate with a powder paint composition.
In the powder paint industry, powder pats are usually electrically charged by means of a corona discharge or tribocharging. Then the powder paint is electrostatically applied to the object to be coated. In general, the electrostatic spraying technique involves use of corona and/or tribo charging guns as described in, for example, Misev xe2x80x9cPowder Coatings Chemistry and Technologyxe2x80x9d, pages 324-344, John Wiley, 1991.
In industrial applications varnishing of, for example, cellulose based sheets and webs is usually carried out using solvent or water-based varnishes. The solvent-based varnishes have as a disadvantage the evaporation of solvents into the atmosphere. Because of environmental reasons emission of solvents has to be reduced drastically. The water-based varnishes have as a disadvantage that they need much energy for drying. To achieve the highest possible gloss the use of calendars is suitable. This separate step is costly and limits the process speed.
Alternatively, liquid UV-curable varnishes are used to overcome these difficulties. However, this group of materials is often rejected because of their sensibilizing and irritating character due to the applied monomers, which are the main components of these coating liquids. Another disadvantage of this coating technology can be that the smell makes the coating unsuitable for food packaging.
GB-A-2056885 discloses a process for coating a cellulose fiber substrate like cardboard with a powder paint composition and curing by radiant heat. Generally, a binder system for a powder paint composition consists of a resin and a crosslinked. A disadvantage of the process according to GB-A-2056885 is the limited capacity of the applied powder paint spray guns which imposes a restriction on the rate of application of powder paint to the substrate. Another disadvantage is that it is very difficult to achieve coatings having a layer thickness below 15 xcexcm.
A further disadvantage is the major thermal load imposed on the heat-sensitive substrate during curing of the powder paint composition.
Powder paints are also being considered for use in the coil coating industry. Coil coating is a special application method in which coiled metal strips are unwound and then passed through pretreating, coating, and drying equipment before finally being rewound. Coil coating allows for very efficient coating of large surface areas in a short time at high throughput. However, the inherent limitation in powder paint spray gun capacity imposes restrictions on the rate of application of powder paints to the coil and on the reliability of the coil coating process. Hence, there is a strong need in this industry for a high speed but yet reliable continuous process for the application of powder coatings.
This appears, for example, from xe2x80x9cCoil Coatings Strategies in changexe2x80x9d by G. C. Simmons Polymers Paint Colour Journal, Jul. 28, 1993, page 373.
An object of the present invention is to provide an improved process for coating a powder paint on a substrate, at rapid rates (e.g. 100 metres of substrate/minute) while yielding a high quality coating.
The process must also offer the possibility to treat large surface areas simultaneously.
The invention is characterized in that powder paint particles are first charged by friction or induction in the presence of magnetic or non-magnetic particles, are next transported and then applied to the substrate, or applied to a transfer medium and subsequently transferred to the substrate, by means of an electric field between the substrate respectively the transfer medium and the means of transport, whereafter the powder paint composition is cured or fused into a powder coating and wherein the substrate is, for instance, metal, textile, plastic wood, board, or paper-like. The powder coating adheres to the substrate.
If a transfer medium is used, the powder paint particles are first applied to the transfer medium by means of an electric field, transported to the substrate by the transfer medium and then applied to the substrate by, for example, electrical, electrostatic or mechanical forces. Thermal processes can also be used in this application step.
The present process allows powder paint particles to be applied to substrates at rates up to, for instance, 200 meters of substrate/minute with film thicknesses of the cured film of between for example, 3 and 200 xcexcm. Moreover, this process satisfies the need to eliminate multiple spray guns in a coil coating process.
The present process allows powder paint particles to be applied to coated or uncoated board- or paper-like substrates such as, for example, cardboard, hardboard, fiber board for example MDF, particle board, chipboard, plywood, veneer, block board (which board products can be paperfaced), wood, timber and paper at rates up to, for instance, 400 meters of substrate/minute with a film thickness of the cured film of between, for example 1 xcexcm and 100 xcexcm.
The process according to the invention results in a substantially (e.g. more than 90%) fully coated substrate. In contrast, in, for example, a printing process the coverage is, for example, only 10% of the substrate.
The charging of the powder paint particles by friction or induction, the transporting or conveying and the application to substrates can be effected using processes commonly known in photocopying technology or laser printer technology (which processes are elucidated in, for example, L. B. Schein, Electrography and, Development Physics, pages 32-244, Volume 14, Springer Series in Electrophysics 1988: the disclosure of which is incorporated herein by reference).
According to a preferred embodiment of the invention the powder paint particles are mixed with magnetic or non-magnetic carrier particles to obtain friction charging whereupon the mixture is transported and the powder paint particles are applied to the substrate by means of an electric field between the substrate and the means of transport.
The powder paint particles may be based on solid particles of a radiation curable binder system.
Suitable radiation curable binder systems include for example a polymer having unsaturated groups and a cationic curable system based on, for example epoxy resins.
Preferably, the radiation curable binder system comprises a polymer having unsaturated groups.
Preferably, said polymer having unsaturated groups has a degree of unsaturation between about 200 and about 1800 grams of polymer per mole of unsaturated group.
Depending on the conductivity of the substrate the electric field between the substrate and the transfer medium or means of transport can be applied with suitable methods like for instance a corona discharge or a moving or fixed counterelectrode (see for instance Schein, pp. 36-37 and 47).
Preferably, the polymer having unsaturated groups has a molecular weight (Mn) between about 800 and about 6800 grams/mole and a viscosity between 1 dpas and 800 dPas (measured with an Emila rheometer at 165xc2x0 C.)
Preferred polymers are unsaturated polyesters, unsaturated polyacrylates and acrylated polyesters.
Preferably, the unsaturated polymer has a degree of unsaturation between 300 and 1800 grams per mole of unsaturated group (WPU), a molecular weight (Mn) between about 800 and about 6800 and a viscosity between 1 dpas and 400 dpas.
The preferred degree of unsaturation for an unsaturated polyester is between 500 and 1500 grams per mole of unsaturated group.
The molecular weight (Mn) is preferably between about 2500 and about 4500 grams/mole. 30 The unsaturated polyester can be (semi)crystalline or amorphous.
In general, the glass transition temperature (Tg) of the amorphous resin is higher than 40xc2x0 C. Semicrystalline resins have a Tg and a melting point (Tm), and Tg is lower than Tm.
The melting point of the (semi)crystalline unsaturated polyester is between about 40xc2x0 C. and about 180xc2x0 C., and preferably, is between about 60xc2x0 C. and about 140xc2x0 C. The melting point and the melting range are discussed at pages 36, 286 and 295 of Misev, Powder Coatings, Chemistry and Technology Wiley 1991, the disclosure of which is incorporated herein by reference.
Thermosetting powder coatings that are based on unsaturated polyester are described in Misev, Powder Coatings, Chemistry and Technology, at pages 167-170, the disclosure of which is incorporated herein by reference.
Preparation of the unsaturated polyester can be carried out in a single-step process in which unsaturated polyfinctional carboxylic acids and glycols are heated to a temperature, for example, between about 180xc2x0 C. to about 230xc2x0 C. for between about 6 hours to about 15 hours.
Preparation of the unsaturated polyester can also be carried out in a two-step process in which saturated glycols and saturated polyfunctional carboxylic acids are esterified in a first step at, for example, between about 230xc2x0 C. to about 250xc2x0 C. for between about 2 hours to about 8 hours. In a second step, saturated or unsaturated glycols and acids, or mixtures thereof, are esterified at, for example between about 180xc2x0 C. and 220xc2x0 C. for between about 2 to about 8 hours.
In general, the unsaturated polyester is obtainable from the condensation of one or more aliphatic or cycloaliphatic mono-, di- or polyfunctional alcohols, or mixtures thereof, one or more aliphatic, cycloaliphatic, or aromatic di- or polyfunctional carboxylic acids, or mixtures thereof, and if desired, a monofimctional carboxylic acid or the corresponding ester of this monofunctional carboxylic acid.
Examples of suitable alcohols and glycols include, among others, benzyl alcohol, ethylene glycol, propylene glycol, neopentylglycol, butanediol, hexanediol, dimethylol cyclohexane, diethylene glycol, glycerol, trimethylol propane, pentaerytritol, dipentaerythritol, or mixtures thereof. Instead of an alcohol or glycol, or together with an alcohol or glycol, one or more epoxy compounds such as, for example, ethylene oxide, propylene oxide, allyl glycidyl ether, or mixtures thereof, can be used.
Examples of suitable di- or polyflimctional carboxylic acids include maleic acid, fumaric acid, itaconic acid, citraconic acid, malonic acid, succinic acid, glutaric acid, adipic acid, sebacic acid, 1,4 cyclohexane dicarboxylic acid, hexahydrophthalic acid, hexachloroendomethylenetetrahydrophthalic acid, dichlorophthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, or mixtures thereof.
Fumaric acid, maleic acid, or a mixture thereof, is preferred. The carboxylic acids can also be applied in the corresponding anhydride form, so.that, for example, tetrahydrophthalic anhydride, maleicanhydride, phthalic anhydride, or mixtures thereof can be used.
If desired, the unsaturated polyester can also be obtained from saturated or unsaturated monofunctional carboxylic acids, or mixtures thereof These monofunctional carboxylic acids include, for example, synthetic or natural fatty acids having 2 to 36 carbon atoms. Corresponding esters of monofunctional alcohols such as glycerol are used for esterification. Examples of suitable monofinctional carboxylic acids include, among others, lauric, stearic, oleic, linoleic, benzoic, acrylic, methacrylic acid, or mixtures thereof. The unsaturated polyester can also contain dicyclopentadiene. If the unsaturated polyester is hydroxyl functionalized, then the hydroxyl number of the polyester is usually between about 18 mg KOH/gram of resin and about 75 mg KOH/gram of resin.
If the unsaturated polyester is acid functionalized, the acid number is usually between about 18 mg KOH/gram of resin and about 75 mg KOH/igram of resin.
Suitable acrylate polymers include, for instance, acrylate polymers having several side chains containing unsaturated groups.
These polymers can be obtained by means of a two-step process. In the first step an acrylate polymer is prepared by a conventional polymerization process, in which also a certain proportion of functional monomer is copolymerized to obtain a functionalized acrylate polymer. This functional monomer, which usually is present in amounts of between 3 and 60 wt. %, can be, for example, an epoxy functionalized monomer such as glycidyl (meth)acrylate; an acid functionalized monomer such as, for instance, (meth)acrylic acid, a hydroxy functionalized monomer such as, for instance hydroxyethyl (meth)acrylate; or an isocyanate functionalized monomer such as, for instance. TMI (metaisopropenyl-xcex1-xcex1-imethylbenzyl isocyanate).
In the second step for the acrylate synthesis, an addition reaction, is carried out between the functional groups of the acrylate polymer which is prepared in the first step and a compound having both a functional group capable of reacting with the functional groups and also an unsaturated group in the side chain.
For the addition reaction of this second step, the functionalized acrylate resin can be dissolved in a solvent such as for example toluene, xylene and/or butyl acetate. The compound having the unsaturated group, is added at temperatures between, for example, 50xc2x0 C. and 150xc2x0 C. and next, stirring takes place for some hours. Progress of the reaction can be monitored by means of titrations of, for instance, acid groups or isocyanate groups. Addition reactions of this second step are, for instance, between a compound having an acid group and an epoxy functional acrylate polymer, a compound having a hydroxyl group and an isocyanate functionalized acrylate polymer, a compound having an isocyanate group and a hydroxyl functionalized acrylate polymer, a compound having an anhydride group and a hydroxyl functionalized acrylate polymer or a compound having an epoxy group and an acid functionalized acrylate polymer.
It is preferred to allow an epoxy functionalized acrylate polymer to react with (meth)acrylic acid.
The unsaturated side chain can have, for example, methacrylate ester, allyl, vinyl, vinyl ether, or anhydride groups, or mixtures thereof. Suitable compounds that are allowed to react with the functionalized acrylate copolymer are, for example, methacrylic acid, glycidyl methacrylate, TMI, allylglycidyl ether, hydroxybutylvinyl ether, maleic anhydride, or mixtures thereof.
The unsaturated acrylate polymer can also be obtained by means of a multi-step process. For example, one of the isocyanate groups of a diisocyanate compound such as, for example, isophorone diisocyanate (IPDI), hexamethylene diisobyanate (HDI) methylene-biscyclohexylisocyanate (hydrogenated MDI) is allowed to react with a methacrylate monomer having the hydroxyl functionality. The resulting urethane compound, which contains isocyanate and methacrylate groups for further reaction, can subsequently be allowed to react with, for example, an acrylate copolymer having the hydroxyl functionality. This acrylate copolymer can be prepared using, for example, hydroxyethyl methacrylate as a functionalized monomer.
Depending on the binder system a crosslinker has to be applied or can optionally be applied.
A preferred crosslinking agent for the radiation curable binder composition according to the present invention is an oligomer or polymer having one or more vinyl ether, allyl, vinyl ester, or (meth)acrylate groups. More preferably an oligomer or polymer having at least two vinylether groups is applied.
The crosslinking agent having vinyl ether groups is preferably made of a divinyl ether functionalized urethane based on, for example, a diisocyanate and hydroxybutyl or hydroxyethyl vinyl ether.
Suitable crosslinking agent having (meth)acrylic groups include for example dimethacrylate functionalized urethane based on, for example, a diisocyanate and hydroxyethyl methacrylate or polymethacrylate functionalised urethanes.
Suitable difunctional isocyanates used in the preparation of the crosslinking agent include isophorone diisocyanate, or methylene diisocyanate, or methylenebiscyclohexylisocyanate. Functionalized oligomers derived from these diisocyanates such as isocyanurates, uretdiones, and biurets can, for example, also be used.
Preparation of vinyl ether functionalized diurethanes can be carried out by means of the process described in, for example, EP-A-505.803, the complete disclosure of which is incorporated herein by reference
Preparation of suitable vinyl ether terminated ester oligomers for use as crosslinking agent is described in WO-A-89/10.346, the complete disclosure of which is incorporated herein by reference. A suitable vinyl ester functional compound is, for example, divinyl adipate.
The crosslinking agent preferably contains between about 1 to 10 vinyl groups, and more preferably, contains at least two vinyl groups per molecule.
The crosslinking agent can be a liquid or a solid. Solid crosslinking agents are preferred because good powder stability is more easily achieved, after combination of the crosslinking agent with the unsaturated polymer.
The crosslinker having allyl groups can for example have 2-12 allyl groups. The crosslinker can have allylether groups and further ester or urethane groups. The crosslinker can also have vinylether and allylether groups. It is also possible to use vinylether and allylether containing crosslinkers.
Very suitable crosslinkers include, for example, the ester of trimethylolpropane diallyl ether or pentaerythritol triallyl ether and a polycarboxylic acid (anhydride). Also oligomers containing allyl groups, for example, allyl alcohol, trimethylolpropane diallyl ether, allyl glycidyl ethers and pentaerythritol triallyl ether. Examples of suitable isocyanates are isophorone diisocyanate, toluene diisocyanate, hexamethylene diisocyanate and methylene biscyclohexylisocyanate.
The relative amounts of polymer having unsaturated groups and crosslinking agent in the binder composition depend in part on the choice of crosslinking agent. In general, if the crosslinking agent is vinyl ether functionalized, the equivalent ratio of polymer unsaturation to crosslinking agent unsaturation is between 1:2 and about 2:1, and preferably, about 1:1. In general, if the crosslinking agent is a methacrylate or a vinyl ester functionalized compound, the equivalent ratio of polymer unsaturation to crosslinking agent unsaturation is between about 1:2 and about 2:1, and preferably, about 1:1.3.
A suitable cationic curable system based on epoxy resins is disclosed in EP-A-667381.
Radiation curing is preferably carried out via ultraviolet (UV) or electron beam (EB) curing. These techniques are described in, for example, the article, xe2x80x9cUV and EB-curingxe2x80x9d, by S. J. Bett et al. in Jocca 1990 (11), pgs. 446-453, the complete disclosure of which is incorporated herein by reference. Most preferably UV-curing is applied. Very suitable UV-lamps for curing thin layers or to obtain high speeds are excimer lamps (as described for example at pages 48-52 of the Conference Proceedings of RAD TECH Europe Sep. 25, 1995, in Maastricht).
For UV-radiation curing of the powder paint formulation a photoinitiator is mixed with the binder composition at a temperature between, for example, about 50xc2x0 C. and about 150xc2x0 C. Mixing can take place in a solvent or in the melt with use of an extruder, wherein the latter is preferred. Pigments, flow promotors, other additives, or mixtures thereof, can also be used.
Another advantage of the present invention is the fast cure at lower temperatures, in comparison with the prior art processes related to thermocuring systems, resulting in a lower thermal load on the substrate which is very relevant for the present substrates.
An important advantage of the radiation curing according to the present invention is that flow of the powder paint on exposure to heat is fully separated from the curing reaction on exposure to UV or electron beam radiation, which enables powder coatings to be made that exhibit excellent flow.
Common additives such as, for example, pigments, fillers, flow promoters, tribo additives, stabilizers, or mixtures thereof can be added to the binder system. Suitable pigments include, for example, inorganic pigments such as titanium dioxide, zinc sulphide or iron and chromium oxide, and organic pigments such as, for example, azo compounds.
Suitable fillers include, for example, metal oxides, silicates, carbonates, sulfates, or mixtures thereof, as known to those skilled in the art.
Upon curing or fusing of a powder paint composition a powder coating is obtained.
The powder paint composition may comprise any one of the known binder systems such as, for example, described by Misev, xe2x80x9cPowder Coatings, Chemistry and Technologyxe2x80x9d (John Wiley and Sons, 1991), pages 9-171, the disclosure of which is incorporated herein by reference.
The powder paint composition can be tribomodified, as described in, for example, U.S. Pat. No. 5,124,387, the complete disclosure of which is incorporated herein by reference.
The powder paint composition can, if desired, comprise usual additives such as fillers, pigments, antioxidants, stabilizers, flow agents, catalysts and triboadditives as described in, for instance, U.S. Pat. No. 5,342,723. The powder paint composition can also, for example, comprise additives for flow control purposes or for charge control purposes as described, for instance, in U.S. Pat. No. 4,960,666.
The powder paint compositions can, for example, be prepared and characterized as described in, for example, the aforementioned Misev on pages 224-283 which disclosure is incorporated herein by reference. The selection of grinding, classifying and sieving equipment is important for obtaining the desired particle size of the powder paint particles. For this purpose target mills, fluidised bed mills, static classifiers and rotating classifiers may be used.
The binder system can be a thermosetting or a thermoplastic system. Preferably, the binder system for the powder paint is a thermosetting binder system.
Various types of curing reactions can be employed in thermosetting powder coatings for instance as disclosed in the afore mentioned Misev and radical curing systems such as UV-curing systems and electron beam curing systems. In many cases the thermosetting powder paint compositions comprise a binder system consisting of a resin and a crosslinker. Suitable resins include, for example, polyester resins, acrylic resins and alkyd resins.
Examples of suitable binders are: saturated carboxylated polyester resin/triglycidylisocyanurate (TGIC), saturated carboxylated polyester resin/epoxy resin, saturated carboxylated polyester resin/crosslinker containing hydroxylamide group, saturated carboxylated polyester resin aliphatic oxirane, saturated hydroxylated polyester resin/isocyanate, polyester resin/diglycidyl phtalate containing crosslinker, saturated hydroxylated polyester resin/hexamethoxy-methylmelamine (HMMM), saturated hydroxylated polyester/glycoluril(derivative), saturated hydroxylated polyester/benzoguanamine (derivative), saturated hydroxylated polyester resin/amino resin, saturated hydroxylated polyester resin/phenolic resin, epoxy resin/amino resin, epoxy resm/phenolic resin, epoxy resin/anhydride, epoxy resin (selfcrosslinking), phenolic resin (selfcrosslinking), epoxy ester resin/amino resin, amino resin/isocyanate, acrylamide resin (selfcrosslinking), acrylic resin/hydroxy-functional compound, unsaturated acrylic resin (selfcrosslinking), unsaturated acrylic resin/vinylether, unsaturated polyester resin/vinylether and saturated epoxidized acrylic resin/dodecanedicarboxylic acid.
Preferred binder systems include a carboxylated polyester as resin and TGIC, an epoxy resin, a hydroxylamide group containing crosslinker (for example PRIMID(trademark)) or an aliphatic oxirane (as disclosed in EPA-600546) as crosslinker.
Other preferred binder systems include hydroxylated polyester resin with HMMM or an isocyanate group containing crosslinker, an epoxy resin with a phenolic resin crosslinker and an epoxy ester resin with an amino resin crosslinker.
Upon curing or fusing of a powder paint composition a powder coating is obtained.
The median particle size (by volume) of the powder paint particles X50,3 (as defined according to the description and notation at pages 12-14 of Mechanische Verfahrenstechnik by Prof. Rumpf (Carl Hansen Verlag, 1975)) can be for example below about 200 xcexcm, and preferably, is between about 5 and about 60 xcexcm.
The selection of the particle size depends on for example the desired final coating thickness for a given application.
The particle size distribution can be as broad as it is in conventional powder paint technology.
Preferably, the particle size distribution is relatively narrow. More preferably, the ratio X75,3:X25,3 less than 3 (according to the definition in the aforementioned Rumpf), since the efficiency of the development step may vary with the particle size.
It is one of the advantages of the process according to the present invention, that it is possible to apply particles having median particle sizes between about 5-30 xcexcm. It is very difficult to apply these particles with conventional spray guns.
Carrier particles can be either magnetic or non-magnetic. Preferably, the carrier particles are magnetic particles.
Suitable magnetic carrier particles have a core of, for example, iron, steel, nickel, magnetite, xcex3-Fe2O3, or certain ferrites such as for example CuZn, NiZn, MnZn and barium ferrites. These particles can be of various shapes, for example, irregular or regular shape.
Generally, these carrier particles have a median particle size between 20 and 700 xcexcm. Preferably, the carrier particle size distribution is narrow and more preferably the ratio X75,3:X25,3 less than 2.
Exemplary non-magnetic carrier particles include glass, non-magnetic metal, polymer and ceramic material.
Non-magnetic and magnetic carrier particles can have similar particle size.
Preferably the carrier core particles are coated or surface treated with diverse organic or inorganic materials to obtain, for example, desirable electrical, triboelectrical and/or mechanical properties. Inorganic materials are described in for example U.S. Pat. No. 4,925,762 and U.S. Pat. No. 5,039,587. Organic coating materials include, for example, polymers having fluor-, silicone-, acrylic-, styrene-acrylic, melamine- or urethane-group. Mixtures of these polymers can also be used. Preferably a fluor containing polymer is used as coating.
For coating the carrier particles, any suitable method to coat a thin or thick layer on a powder surface, such as, for example spray coating in a fluidized bed or dry coating an rotary kiln, can be used.
The carrier coatings can comprise suitable fillers or additives to control for example, triboelectrical, electrical or mechanical properties of the carrier coating. For example conductive materials such as, for example, carbon black and metal powder or charge controlling material and flow improving materials can be used.
The carrier particles may be conductive (as described in for example U.S. Pat. No. 4,076,857) or non-conductive.
For direct application without a transfer medium, on a metal substrate, the carrier particles should be preferably non-conductive and they should have a well-defined high resistivity of, for example, 109-1011 Ohm at 10V potential and a break-through voltage above 1,000V (measured with a c-meter supplied by Epping GmbH).
In case of use of a transfer medium the carrier particles can be conductive or on-conductive.
Preferably, carriers particles having high voltage break through are used so hat high electric fields can be used between transport means and substrate or transfer media to achieve a thick powder layer.
A developer comprises powder paint particles and carrier particles. A development method is a way of developing and a development unit is a complete system comprising of, for example, a developer roller (transport medium), mixing screw(s), a supply device, blades, detectors and the like. Other examples are described in, for example, GB-A-2097701, U.S. Pat. No. 4,147,127 and U.S. Pat. No. 4,131,081.
In the present invention the development method can be either one-component or two-component. According to a preferred embodiment of the invention the two-component development method, in which the carrier particles are mixed with the powder paint particles, is used. Alternatively, also all kinds of monocomponent development like, for example, magnetic conductive, magnetic insulative or non magnetic development as disclosed on pages 203 ff of the above mentioned Schein or binary developers consisting of e.g. magnetic toner and carrier as described in U.S, Pat. No. 5,516,613 can be used.
Preferably, a combination of powder paint particles having a X50,3 below 80 xcexcm and a X95,3 below 120 xcexcm and carrier particles having a X50,3 below 180 xcexcm and a X95,3 below 200 xcexcm is used.
More preferably, a combination of powder paint particles having a X50,3 below 30 xcexcm and above 5 xcexcm and a X95,3 below 50 xcexcm and carrier particles having a X50,3 below 180 xcexcm and above 5 xcexcm and a X95,3 below 200 xcexcm is used.
In the two-component developer the amount of powder paint particles can be, for example, between about 1 and about 50 wt. % and preferably between about 5 and about 25 wt. % (relative to the amount of developer). It is an advantage of the process according to the invention that it is possible to use powder paint concentrations well in excess of 10 wt. %. Consequently, the amount of carrier particles can be between about 50 and about 99% by weight (relative to the amount of developer) and preferably is between about 75 wt. % and about 95 wt. %.
The powder paint concentration can be externally or internally (in the evelopment unit) controlled. External control can be effected by measurement of ayer thickness of uncured or cured powder by, for example, optical, photothermical or dielectrical means. Internal control car be carried out in the developer station by means of powder paint concentration control by any suitable means like inductive control (see, for example, U.S. Pat. No. 4,147,127 and U.S. Pat. No. 4,131,081) or volume control.
In a two-component development method the powder paint particles are triboelectrically charged by intensive mixing and friction with the carrier particles.
In the process according to the present invention it is also possible to use a one component development method with the carrier particles being incorporated in the powder paint particles as disclosed in, for example, U.S. Pat. No. 4,803,143 and U.S. Pat. No. 4,543,312.
In a one-component development method the particles are charged by induction or friction, depending on the selection of the powder paint particles. Both one- and two-component developers can be transported by magnetic, electric and/or mechanical transport.
Preferably, the means of transport is a conveying method.
The mixture is preferably conveyed by means of a magnetic roller as described in for example U.S. Pat. No. 4,460,266. In addition to a magnetic brush apparatus also useful in the present process are, for example, non-magnetic cascade development (see page 94 of the hereinbefore incorporated reference of L. H. Schein), magnetic cascade development (as described in for example xe2x80x9cThe 9th International Congress on Advances in Non-Impart Printing Technologies/Japan Hardcopy ""93, pages 133-140). In addition also transport by air, for example, powder cloud development can be used. An exemplary process is described in for example U.S. Pat. No. 2,725,304. Also jumping (projection) development (see for example Electrophotography, Fourth International Conference, pages 207-228. Nov. 16-18, 1981) can be carried out.
When two-component developers are used, the parameters which are relevant for the process (such as, for example, powder paint concentration, development potential and machine parameters) can be chosen depending on the application. This may lead to batch developer replacement, e.g. after certain time intervals or if certain parameters are out of a control range. Preferably, continuous developer material replenishment, as described in, for example. U.S. Pat. No. 4,614,165, can be used to avoid process interruption for batch replenishment.
Suitable substrates to be coated include, for instance, metal (such as for example aluminum, steel, electrogalvanised steel and hot dip galvanized steel), textile, plastic and wood. Preferably, the process according to the invention is effected with use of coilcoating substrates. The preferred substrate is metal.
The substrate can have any desired shape such as band or sheet, i.e. continuous or discontinuous.
The substrate may be magnetic or non-magnetic. If a magnetic substrate and a magnetic developer are used, generally an intermediate transfer medium like a belt or a roller has to be placed between the magnetic roller and the substrate as for example disclosed in EP-A-354530.
If a transfer medium is used between the transport means and the substrate any suitable transfer medium like metallic drums, dielectric drums, foil from metals or polymers like silicone rubber belts, as described in xe2x80x9cProceeding of ISandT""s Seventh International Congress on Advances in Non Impact Printing Technologiesxe2x80x9d, Vol. 11, page 453-462, or composite materials can be used.
Alternatively a two step transfer may be used in which a layer of powder paint is developed on a first transfer medium and then transferred to a second transfer medium and finally transferred to the substrate.
The present coating process can be advantageously used in the application of varnishes.
The process according to the present invention is preferably used in a coil-coating process or in a sheet-coating process. The coated substrates obtained with this sheet coating process can advantageously be used in the production of cans. Surprisingly, it was found that fully closed layers having a thickness of about 5 xcexcm can easily be obtained with the process according to the invention whereas it is very difficult to obtain such layers with spraying techniques.
The process can also be used in a foil-coating process and furthermore in any process where a constant layer of powder paint has to be applied to a surface.
The pretreatment of substrates is important. Suitable pretreatment processes are described in, for example, xe2x80x9cPretreatment Processes and Materialsxe2x80x9d 1986 European Coil Coating Directory, pages 134-135.
The substrate can be pretreated or coated with, for instance, a primer. Such a primer may be coated as a thin layer in wet form or by means of the process of the present invention.
Generally, more layers can be applied with or without intermediate fusing eparately on the substrate or separately on the transfer medium and jointly transferred to the substrate.
A preferred process according to the present invention thus comprises charging of the powder paint particles by intensive mixing and friction with magnetic carrier particles, transport of carrier particles and powder paint particles with the aid of a magnetic roller and subsequent application of the powder paint particles to a substrate by means of an electric field between the substrate and the magnetic roller, whereafter the powder paint composition is made to cure or to fuse to a powder coating using techniques known in the art.
In this preferred process it is also possible to apply a transfer medium as described previously.
For melting, the applied powder paint can be transferred into an oven or can be exposed to IR radiation, or a combination of both. Alternatively all other suitable fusing methods e.g. as disclosed on page 47-49 of the above mentioned Schein can be used. The applied powder paint is molten at temperatures between, for example, about 60xc2x0 C. and about 140xc2x0 C. to form a continuous, smooth, softened or molten coating with coating thickness between, for example, about 1 xcexcm and about 100 xcexcm. Then, the hot panel is cured with use of a UV light source. Post-heating can also be carried out. The powder coating adheres to the substrate. It is possible to apply the coating on printed substrates produced with common printing techniques (offset, gravure, flexo, digital printing either electrographic or inkjet). The printing process and the coating process can be carried out in two separate steps (off-line) or as well in one production step (on-line).
The coating may cover the full area of the substrate or only part of it (spot coating). In case of spot coating any image creating technology as described in the above mentioned Schein may be used to create an image on a suitable transfer medium or as first transfer medium in case of two step transfer as described before.
The carrier particles may be dosed to a mixing arrangement in which one or more means of intensive mixing such as, for example, worm wheels are present along with a magnetic roller. Suitable mixing arrangements are described in, for example, xe2x80x9dProceeding of ISandT""s Seventh International Congress on xe2x80x9cAdvances in Non Impact Printing Technologiesxe2x80x9d, Vol. 1, pages 259-265, the complete disclosure of which is herein incorporated by reference.
Next, an amount of powder paint particles is fed into the mixing arrangement which is selected so as to obtain a powder paint particles concentration of, for example, about 5-15 wt. % powder paint relative to the amount of carrier particles. In this way, a developer is formed.
During the intensive mixing and friction in the mixing arrangement, due to the action of the worm weels, the carrier particles and the powder paint particles become oppositely (tribo) electrically charged. A layer of electrically charged powder paint particles covers the carrier particles. The carrier particles subsequently act as carrier for the charged powder paint particles. Next, the developer is fed to the magnetic roller, in which a brush-like structure, also known as the magnetic brush, is formed. The magnetic roller transports the brush to the contact area with the substrate or transfer medium. In this way, powder paint particles, as well as carrier particles, become available at the contact area with the substrate or transfer medium. By applying a sufficiently strong electric field between the magnetic roller and the substrate or transfer medium the powder paint particles can be drawn from the brush to the substrate or transfer medium, onto which they adhere electrostatically. In the case a transfer medium is used, the powder paint particles are subsequently transported to the contact area of the transfer medium and the substrate and next transferred to the substrate via one of the aforementioned transfer processes.
Finally, the magnetic brush is scraped off the magnetic roller as it is returned into the mixing bin.
As a result, a layer of powder paint particles forms on the substrate, which layer is substantially free from carrier particles, and can then be cured. Eventually an apparatus which can catch carrier particles can be introduced such as for example a catching equipment for residual carrier, as described in xe2x80x9cProceeding of ISandT Eighth International Congress on Advances in Non Impact Printing Technologiesxe2x80x9d, pages 391-393. The thickness of said layer of powder paint particles can, for example, be controlled via the electric field strength between the magnetic roller and the substrate, or the transfer medium, the magnitude of the charge on the powder paint particles (e.g. by varying the concentration and the mixing time) and the roller speed.
The process according to the invention can advantageously be applied in equipment for small-scale application of overprint varnishes. These machines can be used online or offline in connection with black and white, two colour or full colour copy machines or computer output printers to produce paper or cardboard sheets with high gloss and high resistance e.g. against fingerprints, oil and acids. The process may be used as an alternative to lamination.
The process can also be used in-line or off-line in connection with printing machines of any speed and any technology (for example large scale) like offset, gravure, flexo or digital printing to produce overprint varnishes e.g. for cover sheets of booklets and printed cardboard for packaging.
The process may be used for varnishes, which may be applied below or above a tonerbased image to improve adhesion of the toner to the paper and to improve protection against, for instance, falsification of a document.
The development station with toner hopper and optional the transfer means and the fusing and curing may be designed in a way that it can be easily interchanged, for instance as a cartridge, by any (unskilled) person, for example, for use in a copying machine or laser printer. Consequently, these small scale overprint varnish application machines can be used, for example, in printing shops, copy shops and offices.